Speed control observer applied to speed control system for motor

Abstract: A speed estimation observer for a motor control system outputs a motor speed estimate (i) in a motor speed control system upon receiving a torque command (τ M *(i)) and an averaged motor speed value n M (j) . The speed estimation observer includes a time lag correction value calculating block for implementing a correction of a model output speed estimate. Therefore, the accuracy and stability of the speed estimation are improved, and the control system keeps the stability in a speed range of low to high speed and ensures the disturbance suppressing effect in a high speed range.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvements in a speed estimation observer apparatus for a motor speed controlling system using a rotary encoder and operated in an extremely low speed range. 2. Description of the Background Art Generally, in a speed control system for controlling a speed of a motor by using a rotary encoder of a relatively low resolution, when the motor is rotated in an extremely low speed, an encoder pulse interval becomes longer than a speed control period, and therefore accurate speed information cannot be obtained during such a speed control period. Such a relationship between the speed control period and encoder pulse is shown in Fig. 9. In Fig. 9, Ts is a speed control period, Tp is a period of a pulse encoder, and Td is a difference between Ts and Tp. Accordingly, when the encoder pulse is inputted, an average value of a speed is derived by using a pulse period Tp from the following equation: n M = 60/pp·1/ T p where pp is the number of pulses per rotation of the encoder [P/R], Tp is an encoder pulse period [sec], and n M is an average value of the rotation speed of the motor [rpm]. As is clear from the above equation, in an extremely low speed range, that is, when the encoder pulse period becomes extremely large, the average speed becomes indefinite. Accordingly, the speed control system tends to be unstable, and responsibility thereof is degraded. In order to solve such difficulties, there is proposed a control system in which a speed during an interval between adjacent encoder pulses is estimated by using a speed estimation observer apparatus. Such a control system is shown in Fig. 11. A speed estimation observer apparatus 10 applied to the control system is shown in Fig. 10 and includes a load torque observer means of a minimum dimension. As shown in Fig. 10, a deviating means 110 receives a torque command τ M * (i) and a load torque estimate , and outputs a deviation output therebetween. The deviation output is supplied to a first calculating means 120. The first calculating means 120 includes a division means 120a in which the speed control period Ts is divided by a model machine time constant T M *, an integrator 120c, and an adder 120b which adds an output of the division block 120a to an output of the integrator 120c. The model output speed estimate derived in the first calculating means 120 is supplied to a second caculating means 130. The second calculating means 130 derives an average value during pulse interval and outputs the calculated result to a plus input end of a first deviating means 140. A minus input end of the first deviating means 140 receives an average value n M (j) which is of a speed detection output from the pulse encoder 15. The deviation output of the first deviating means 140 is supplied to an observer gain means 160 and multiplied by a predetermined (generally proportional) gain. The multiplied value is outputted as a load torque estimate to the deviating means 110. In addition, the deviation output of the first deviating means 140 is supplied to a minus input end of a second deviating means 170. A pulse input end of the second deviating means 170 receives the model output estimate . The second deviating means 170 outputs the speed estimate . Since in the speed estimation observer shown in Fig. 10 the speed estimate and the load torque estimate are simultaneously obtained, a disturbance compensation is carried out for obtaining a disturbance suppressing effect. As shown in Fig. 11, the speed estimation observer means 10 outputs a speed estimate and a load torque estimate . The speed estimate and the speed set value τ M * (i) are supplied to a minus input end and a plus input end of a third deviating means 18, respectively. The deviation output therefrom is supplied to a speed amplifier 19 of a proportion gain K WC . An adder 20 adds the output from the speed amplifier 19 and the load torque estimate , and outputs a torque command τ L *. A fourth deviating means 21 receives the torque command τ L * and the actual load torque τ L , and outputs a deviation therebetween. The deviation is supplied to a motor 22 for its control. A speed detection means 23 receives a signal from a pule encoder 15 and outputs the averaged value to the speed estimation observer apparatus 10. The manner of operation of the speed estimation observer apparatus 10 applied to a motor speed control system shown in Figs. 10 and 11 will be discussed. A model output estimate is obtained in a manner that the deviation between the torque command τ M * (i) and the load torque estimate is integrated with respect to the observer model machine time constant T M *. Following this an average speed value during each pulse interval Tp is obtained from the model speed output estimate , and a deviation between the average speed value n M (j) and the average speed value is obtained. Multiplying the deviation by the observer gain (g), the load torque estimate is obtained. Then, the speed estimate is derived by subtracting the deviation at the first deviating means 140 from the observer model output estimate . The derived speed estimate is supplied to the speed control amplifier 19 for executing the control of the motor 22. Also, it becomes possible to execute a load disturbance compensation by obtaining the torque command upon adding the load torque estimate and the output of the speed amplifier 19 in the adder 20. However, if the observer gain is set at a relatively large value for obtaining the disturbance suppressing effect in a condition that an encoder pulse interval is smaller than a speed control period, such a large observer gain tends to invite a unstableness of the speed control system in a condition the encoder pulse interval is longer than the speed control period. Therefore, the observer gain should not be set at a large value. However, on the other hand, if the observer gain is set at a relatively small value, the disturbance suppressing effect in a high motor speed range may be degraded. A speed estimation observer apparatus of the kind as depicted in Fig. 10 and as described above is disclosed in EP-A-514847. SUMMARY OF THE INVENTION It is an object of the present invention to provide a speed estimation observer apparatus for a motor speed controlling system which apparatus enables the switching of an observer gain to be carried out without generating the transient phenomena of the controlling system. This object is achieved by the speed estimation observer apparatus as specified in claim 1. Advantageous developments of this apparatus are specified in the subclaims. A speed estimation observer apparatus according to the present invention and as claimed is applied to a motor speed controlling system. The speed estimation observer apparatus is arranged to receive a torque command and an averaged motor speed value detected by a speed detector and to output a speed estimate. The speed estimation observer apparatus comprises a first calculating means which receives a deviation between the torque command and a load torque estimate and outputs a model output speed estimate by integrating the deviation with respect to an observer model machine time constant. A second calculating means receives a corrected model output speed estimate and outputs an averaged model output speed estimate during a pulse interval of the speed detector. A first deviating block receives the averaged model output speed estimate from the second calculating means and the averaged motor speed value detected by each speed detected period of the speed detector, and outputs a first deviation between the averaged model output speed estimate and the averaged motor speed value. An observer gain means receives the first deviation and outputs a load torque estimate by multiplying the first deviation with an observer gain. A second deviating block calculates a second deviation between the corrected model output estimate and the first deviation. A difference calculating block calculates a difference between two speed deviations of adjacent two speed detected periods and supplying the difference for adding the reference to the model output estimate outputted from the first calculating means for obtaining the corrected model output estimate. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, like reference numeral designate like part and elements throughout all figures, in which: Fig. 1 is a circuit block diagram of an embodiment of a speed estimation observer apparatus according to the present invention; Fig. 2 is a circuit block diagram of another embodiment of the speed estimation observer apparatus according to the present invention; Fig. 3 is characteristics graphs showing the results of the simulation of the speed estimation observer apparatus according to the present invention of Fig. 1; Fig. 4 is characteristics graphs showing the results of the simulation of a model where only the observer gain was switched and other correction was not carried out; Figs. 5, 6 and 7 are explanatory views for explaining averaging processes; Fig. 8 is a circuit block diagram of another embodiment of the speed estimation observer apparatus according to the present invention; Fig. 9 is a signal timing chart for explaining a relationship between a speed control period and encoder pulse; Fig. 10 is a circuit block diagram including a conventional speed control observer; and Fig. 11 is a circuit block diagram showing a motor speed control system to which the speed estimation observer is applied. DETAILED DESCRIPTION OF THE INVENTION Referring now to Fig. 1, there is shown an embodiment of a speed estimation observer apparatus according to the present invention. The embodiment shown in Fig. 1 is generally similar to that of Fig. 10 except that a difference calculating means 31 is further provided. The explanation of the other part except for the difference calculating means 31 is similar to that of Fig. 10. Furthermore, the speed estimation observer apparatus according to the present invention is applied to the control system as shown in Fig. 11. As shown in Fig. 1, a deviating means 11 receives a torque command τ M * (i) and a load torque estimate , and outputs a deviation output therebetween. The deviation output is supplied to a first calculating means 12. The first calculating means 12 includes a division means 12a in which the speed control period Ts is divided by a model machine time constant T M *, an integrator 12c, and an adder 12b which adds an output of the division means 12a to an output of the integrator 12c. The model output speed estimate derived in the first calculating means 12 is supplied to a second calculating means 13. The second calculating means 13 derives an average value during a pulse interval and outputs the calculated result to a plus input end of a first deviating means 14. A minus input end of the first deviating means 14 receives an average value n M (j) which is of a speed detection output from the pulse encoder 15 as shown in Fig. 11. The deviation output of the first deviating means 14 is supplied to an observer gain means 16 and multiplied by a predetermined (generally proportional) gain. The multiplied value is outputted as a load torque estimate to the deviating means 11. The difference calculating means 31 comprises an observer gain inverse-number means 32, a deviating means 33 and an adder 34. The observer gain inverse-number means 32 receives the predetermined gain from the observer gain means 16 and switches the observer gain (g j+1 ) at a next sampling time (j+1). The deviating means 33 which receives an multiplied output from the observer gain inverse-number means 32 and a deviation output of a first deviation means 14 to its plus and minus input ends, respectively. The adder 34 receives a deviation output of the deviation means 33 at its first end, and an integrator 35 which integrates the output of the adder 34 and supplied it to a second input end of the adder 34. The multiplied output of the observer gain inverse-number means 32 is supplied to a minus input end of the second deviation means 17. The output Δn c ' (j) (integral element) and the output of the adder 12b are added in an adder 36. Hereinafter, the process for constructing the speed estimation observer apparatus according to the present invention will be discussed. When the load torque τ L is applied to the speed estimation observer apparatus according to the present invention, a speed deviation Δn (j) is generated between an average value of an actual motor speed n M(j) and either of a model output speed estimate or an averaged model output speed estimate . The speed deviation is derived from the following equation: Consider the change of the observer gain (g j ) to (g j ') at a time j, wherein it is assumed that an estimation of the load torque has already finished and is converging to a predetermined value . Further, it is assumed that the load torque does not varied during the switching of the observer gain and therefore In case that the speed deviation Δn (j) during a period between the time (j-1) and the time (j) is obtained at the time (j) from the equation (1), it is impossible to switch the observer gain from g j to g j ' at the time (j) without changing the relationship The reason is because the following equation must be satisfied: wherein τ and L(j) , g j and are constant. Therefore, consider to delay the switching of the gain at a next time (j+1). That is, at the time (j) (g j ) is used as an observer gain, and the observer gain is switched as (g j ')= (g j+1 ) at the time (j+1). Assuming that the observer gain is switched from (g j ) to (g j+1 ) at the time (j+1), the speed deviation Δn (j+1) at the time (j+1) can be expressed by the following equation (2): If the load torque is not varied, the speed deviation Δn (j) at the time (j) is kept constant. However, if the switching of the gain is carried out at the time (j+1), it is necessary that the speed deviation Δn (j) is corrected with respect to the difference between the speed deviation Δn (j+1) at the time (j+1). Such a difference between the speed deviations at the time (j+1) and the time (j) is derived from the equations (1) and (2) as follows. where it is assumed that Therefore, the following equation (4) is obtained. where since the speed deviation Δn (j) has been integrated and summed to the observer model output estimate , it is necessary to correct the observer model output estimate with respect to the difference of the equation (3). Fig. 1 shows a speed estimation observer apparatus taking the switching of the observer gain into consideration. Since a correction means of the speed deviation Δn (j) is included in the difference calculating means 31, it becomes possible to switch the observer gain during a calculation in the speed control period j. The calculation at the time (j) is carried out as follows: (1) calculating (2) assuming that the observer gain is switched from (g j ) to (g j+1 ) at the sampling time (j+1), and executing a calculation of the equation (3), (3) in order to hold a correction data An at a gain switching time, executing a calculation of the integral element (multiple of the difference), and obtaining a correction data Δn c ' (j) , (4) in order to correct by Δn c ' (j) , inputting to the minus input end of the deviation means 17. The above-mentioned operation in paragraph (4) is carried out by the following reasons: Because the observer gain is switched to (g j+1 ) at the time (j+1), by adding Δn c ' (j) to which is adjusted so as to correspond to the observer gain (g j+1 ) during a period between the time (j) and the time (j+1). Therefore, it is necessary to set Fig. 2 shows a second embodiment of the speed estimation observer apparatus according to the present invention. The second embodiment is generally similar to the first embodiment except the integral element for calculating Δn c ' (j) is moved to the side of the speed control period. That is, as shown in Fig. 2, the output Δn c ' (j) of the deviating means 33 is supplied to the adder 36 through a switch 37 which is turned on only at a first time (i) next of the time (j). Figs. 3 and 4 show the results of simulation relating to the above mentioned embodiment. Fig. 3 shows the result of the simulation of the embodiment of the speed estimation observer apparatus according to the present invention. Fig. 4 shows the simulation result of a model in which only the observer gain is switched and the corrections of Δn c ' (j) does not executed. These simulations were carried out under the following conditions: (a) A set speed value n M * was changed from 0 to 1/2000 PU (1/2000 of 1500 rpm). (b) The load torque τ L was set at 100PU (τ L = 100PU). (c) The switching of the observer gain was executed 50→100→50 within o.4 -0.6 second. (d) The motor machine time constant T M was set at 2 seconds (T M = 2 seconds). (e) The observer model machine time constant T M * was set at 2 seconds (T M * = 2 seconds). (f) Speed response ω c = 50 (deg./sec.) (g) Encoder condition: 1500 rpm rated speed. 2000 P/R, quadruple signal. In Fig. 3, graph A is a set speed n M *, graph B shows a load torque τ L , graph C shows the observer gain g, graph D shows a torque command τ M , graph E shows a load torque estimate , graph F shows an actual speed n M , graph G shows a speed estimate , graph H shows a speed selected value n M . As is clear from Fig. 3, even if the observer gain is switched during a time period from 0.4 second to 0.6 second, the torque command, load toque estimate, motor actual speed, speed estimate and the speed detected value are almost not varied and are stable. In contrast, as shown in Fig. 4, in case of the model in which only the observer gain is switched and the corrections of the Δn c ' (j) does not executed, the torque command and the like occur transient phenomena. Fig. 5 shows an explanatory view of a method for deriving the average value n M ' (j) . It is noted that for explanation purpose the time between j and (j+1) will be explained. It is also noted that as appreciated from Fig. 5, the average value n M ' (j) is derived from the average value for each speed control period whose total sum of areas is divided by a time. First, the average value from the time (j,0) to (j,n) is derived from the following equation: where , --- , and denote model output estimate of (j,0) through (j,n) number; and denotes the average value. Next, the average value during a time lag (Ts-ΔT Ej-1 ) between the encoder pulse signal j and the speed control period signal i will be derived. Until the encoder pulse is inputted at the time (j+1) as shown in Fig. 6, the torque estimate estimated at the time j is used for estimating the model output estimate . Accordingly, the average value n M ' (j+1) at the time (j+1) is derived from the following equation: The average value during the time period ΔT Ej is derived by using a torque command τ M jn * in the order of the (j,n) number from the following equation: Since the torque estimate at the time (j+1) is obtained by the input of the encoder pulse at the time(j+1), the average value is derived from the following equation: Referring to Fig. 7, the average value during the time period (T S -ΔT Ej-1 ) can be derived from the following equations: As described above, the average value of the model output estimate during the time period Tp (j+1) can be derived from the following equation: When the observer model output average value is calculated from the equation (12), the deviation between the timing between the speed control period signal i and speed detection period signal j can be compensated. Further, in the low speed range, a variable gain Kc at a time no encoder pulse is not derived during the speed control period can be obtain from the following equation: K C = T S (T S -ΔT Ej-1 )+n·T S +ΔT Ej The equation (13) represents a ratio between the encoder pulse interval T P(j+1) and speed control period T S , and the observer gain becomes reduced as the speed becomes low. It is, then noted that if approximately the deviation of the timing between i and j is neglected, the equation (13) may be represented by the following equation: K C = 1/n Referring to Fig. 8, there is shown a circuit block diagram of another embodiment of the speed estimation observer apparatus according to the present invention. In Fig. 8, an averaging process means 100 is used instead of the second calculating means 13 in Fig. 1. The averaging process means 100 calculates the equation (12). The averaged output from the averaging process means 100 is supplied to the plus input end of the first deviation means 14, the minus input end thereof receiving the average value n M(j) of the speed detection output detected form the pulse encoder. The deviated output of the first deviating means 14 is supplied to the variable gain (Kc) means 101 to execute the calculation of the equation (13). As the speed becomes low, the gain is reduced so that the observer gain is supplied form the variable gain means 101 to the observer gain means 16. Thereafter, the same operation as in the first preferred embodiment is carried out. With the thus arranged system, it becomes possible to eliminate error due to a time lag between the speed control period and the speed detection period. This improves the accuracy and stability of the speed estimation. Furthermore, since the variable gain is applied to the speed estimation observer apparatus, it becomes possible to keep the stability of the control system within a range from a low speed to a high speed to ensure the disturbance suppressing effect in a high speed range.

Claims: A speed estimation observer apparatus for a motor speed controlling system, said speed estimation observer apparatus receiving a torque command (τ M *( i )) and an averaged motor speed value ( n M ( j )) detected by a speed detector, and outputting an estimate of the speed ( n and M ' ( i )) , said speed estimation observer apparatus comprising: a first calculating means (12) receiving a deviation between the torque command ( τ M * ( i )) and a load torque estimate (τ and L ( j )) and outputting a model output speed estimate by integrating the deviation with respect to an observer model machine time constant ( T M * ) during each period of the speed control; a second calculating means (13) receiving a corrected model output speed estimate ( n and M ' ( i )) and outputting an averaged model output speed estimate ( ( j )) during a pulse interval of the speed detector; a first deviating means (14) receiving the averaged model output speed estimate ( ( j )) from said second calculating means (13) and the averaged motor speed value ( n M ( j )) detected during each speed detected period of the speed detector and outputting a first deviation between the averaged model output speed estimate ( ( j )) and the averaged motor speed value ( n M ( j )) ; an observer gain means (16) receiving the first deviation and outputting the load torque estimate (τ and L ( j )) by multiplying the first deviation with an observer gain ( g j ) during each period of the speed detected period; a difference calculating means (31) calculating a difference between two speed deviations of adjacent two speed detected periods (Δ n c ( j ); Δ n c '( j )) and supplying the difference to an adder (36) which sums the difference to the model output estimate outputted from said first calculating means (12) for obtaining the corrected model output estimate ( n and M ' ( i )) , said difference calculating means (31) calculating the speed deviation (Δ n j ) during each next period of the speed detection by multiplying the load torque estimate (τ and L ( j )) by an observer gain inverse number (1/ g j +1 ); and a second deviating means (17) calculating a second deviation between the corrected model output estimate ( n and M ' ( i )) and the speed deviation (Δ n j ) during each next period of speed detection for outputting the estimate of the speed ( n and M ' ( i )) during each period of the speed control. The speed estimation observer apparatus as claimed in claim 1, wherein said difference calculating means (31) includes an observer gain inverse-number means (32) for switching an observer gain (1/ g j +1 ) at a next sampling time; a third deviating means (33) which receives an integrated output from the observer gain inverse-number means (32) and the first deviation to its plus and minus input ends, respectively; and an adder (34) which receives the third deviation at its first end, and an integrator (35) which integrates the output of the adder (34) and supplies it to a second input end of the adder (34), the integrated output of the observer gain inverse-number means (32) being supplied to a minus input end of said second deviating means (17). The speed estimation observer apparatus as claimed in claim 1, wherein said adder (36) receives the model output speed estimate and the difference of said difference calculating means (31) and outputs the sum of said estimate and said difference to said second calculating means (13) said second deviating means (17). The speed estimation observer apparatus as claimed in claim 3, wherein the third deviation of the third deviating means (33) is supplied to the adder (36) through a switch (37) which is turned on only at a first control time in next of the speed detected time, the output of the adder (36) being feedbacked to an integrator (12 c) of said first calculating means (12). The speed estimation observer apparatus as claimed in claim 1, wherein said observer gain means (16) is arranged to change the observer gain ( g j ) from a first predetermined value to a second predetermined value when the motor speed becomes larger than a predetermined speed. The speed estimation observer apparatus as claimed in claim 5, wherein said observer gain means (16) includes an observer gain ( g j ) which is a multiple of a fixed gain and a constant. The speed estimation observer apparatus as claimed in claim 5, wherein said observer gain means (16) includes an observer gain ( g j ) which is a multiple of a fixed gain, a constant and a compensation gain.

Inventors: YAMADA TETSUO, YAMADA, TETSUO, YAMADA, TETSUO C/O K.K.MEIDENSHA
Applicants: MEIDENSHA ELECTRIC MFG CO LTD, KABUSHIKI KAISHA MEIDENSHA
Application year: 1994
Main classification: H02P5
IPC codes: H02P29, H02P23, G01P3, G05D13